Stock market is one of the most commonly investment environment used for investment in the world. In the last few years the number of individual investors increased rapidly, unfortunately most of them does not use the investment fundamentals. Although, the huge amount of historical data for stock markets, it is not easy to predict the stock behavior using traditional techniques if we considered the unusual factors such as political, economic and psychological impact factors especially in Egypt. In this study we use data mining techniques to detect stocks behavior within the weekdays using Ezz Steel Company as case study; we find a new knowledge of the relation between stocks prices and the weekdays.